It seems you can “catch” obesity the way you do a cold. When thin animals are infected with adenovirus-36, most get fat, without eating more. The virus has the same effect in people, says Richard Atkinson, MD, director of the Obetech Obesity Research Center in Richmond, Virginia. He collected blood samples from 502 people and found that about 30 percent of the obese people had antibodies to Ad-36, meaning they’d been exposed to the virus at some point. They weighed, on average, 50 pounds more than those who hadn’t been infected. Just 11 percent of thin people had antibodies.

When Ad-36 enters fat cells, it stimulates fat production and instructs cells to store fat faster. About 15 percent of us have been infected by Ad-36 (which can cause pinkeye, diarrhea and a stuffy nose) and are fat or gaining weight because of it, says Dr. Atkinson. He hopes to find an antiviral drug to fight the virus’s fat-inducing effects.